Optical systems for use in head-mounted display systems preferably have the following features:
(1) a sufficiently long eye relief to allow for comfortable viewing by a user wearing eyeglasses;
(2) a sufficiently large exit pupil to minimize restrictions on placement of the observer's eye; and
(3) a sufficiently large field of view to provide a comfortably magnified image of the system's microdisplay.
In addition, it is also desirable for the optical system to be light in weight and to fit into an overall package which can be comfortably worn by the user.
Most frequently the microdisplay used in a head-mounted display system is a LCD light valve device. To assure maximum contrast of the image for this type of device, the optical system used to produce a magnified image of the microdisplay is preferably telecentric on its short conjugate side, i.e., the side where the microdisplay is located.
As known in the art, telecentric lenses are lenses which have at least one pupil at infinity. In terms of principal rays, having a pupil at infinity means that the principal rays are parallel to the optical axis (a) in object space, if the entrance pupil is at infinity, or (b) in image space, if the exit pupil is at infinity.
In practical applications, a telecentric pupil need not actually be at infinity since a lens having an entrance or exit pupil at a sufficiently large distance from the lens' optical surfaces will in essence operate as a telecentric system. The principal rays for such a lens will be substantially parallel to the optical axis and thus the lens will in general be functionally equivalent to a lens for which the theoretical (Gaussian) location of the pupil is at infinity.
Accordingly, as used herein, the terms “telecentric” and “telecentric lens” are intended to include lenses which have a pupil at a long distance from the lens' elements, and the term “telecentric pupil” is used to describe such a pupil at a long distance from the lens' elements. For the lens systems of the invention, the telecentric pupil distance will in general be at least about two times the lens' focal length.